1. Technical Field of the Invention
The invention relates to the technical field of composites based on carbon nanotubes or nanofibers of large specific surface, intended for use as catalyst supports or as a catalyst for the chemical or petrochemical industry, in motor vehicle exhaust gas purification, or in satellite propulsion systems. Their advantage is that of combining the intrinsic properties of the base materials of carbon nanotubes or nanofibers with those of easily manipulated macroscopic structures.
2. Description of Related Art
The catalysts at present used in the fields of the chemical or petrochemical industry or in motor vehicle exhaust gas purification are essentially in the form of grains, extrusions, cylinders, or monoliths. These materials in the form of grains, extrusions, cylinders or monoliths may fulfill a catalyst support function, in which case an active phase is applied to the said support for forming the catalyst. This active phase is often constituted of metals or metal oxides. The said materials may also themselves show catalytic activity, and in this case they constitute the catalyst. The search for new catalysts which are more selective, of better performance, more durable, and more practical to utilize, concerns both the supports and the active phases.
Nanostructured compounds (mean diameter typically varying between 2 and 200 nm) based on carbon, such as nanotubes or nanofibers, have on the one hand great intrinsic mechanical strength, and on the other hand a large external exchange surface and a good interaction with the deposited active phase, permitting a strong dispersion of the latter. These new materials thus have physico-chemical properties of interest for their use in various fields such as catalysis or in reinforcing materials. According to the prior art (see the article by N. M. Rodriguez, A. Chambers and R. T. K. Baker, “Catalytic engineering of carbon nanostructures”, Langmuir Review, vol. 11, pp. 3862-3866, 1995), these carbon-based nanostructured compounds are deposited from a gaseous phase containing ethylene, or a CO—H2 mixture, on a substrate constituted either by a metallic powder or by a solid silica support impregnated with an aqueous solution of iron nitrate which is then calcined and reduced to iron to form an active phase. In both cases, the metal (copper or iron) acts as a catalyst for the formation of nanotubes or nanofibers from a vapor phase. Patent Application WO 01/51201 (Hyperion Catalysts International) gives other methods for preparation of carbon nanotubes and nanofibers and indicates their possible use as catalysts.
Methods of manufacturing nanotubes with a single wall are also known, in which the nanotubes are deposited by vapor deposition on an aerogel of alumina having a specific surface (of the order of 600 m2/g) comprising a growth catalyst of Fe/Mo type (see the article “A scalable CVD method for the synthesis of single-walled carbon nanotubes with high catalyst productivity” by Ming Su, Bo Zheng and Jie Liu, Chemical Physics Letters 322, pp. 321-326 (2000).
Problem Posed
According to the state of the art, carbon-based nanostructured composites are synthesized only with low yields. Furthermore, their nanometric size renders their shaping and use difficult, and gives rise to problems of powder generation during transport and loading; it likewise makes their use impossible in fixed bed reactors, due to charge loss problems. Consequently, not only is the cost price of these compounds high, but also their use as catalyst or catalyst support in industrial processes is difficult and of little effectiveness.
The present invention has as its object to propose new composites based on carbon nanotubes or nanofibers which retain the advantages of these nanotubes or nanofibers, namely their ability to act as a support of a catalytically active phase, and their intrinsic catalytic activity, without having the known disadvantages of the said nanotubes or nanofibers, namely the difficulty of shaping them, the generation of dust, the difficulty of using them in a fixed bed reactor, and their cost.